1. Field of the Invention
This invention pertains to the field of purifying fluid streams by the removal of carbon dioxide and at least sulfur compounds therefrom. More particularly, the present invention relates to an integrated process which involves the utilization of a primary adsorption for the selective adsorption of carbon dioxide and a process for the removal of mercaptan sulfur compounds from the fluid stream, which process provides for lower disposal costs and lower operating costs.
2. Discussion of Related Art
Many hydrocarbons contain sulfur in the form of mercaptans (thioIs). Mercaptans are almost invariably present in refinery fuel gas, LPG, cracked gasolines, straight run gasolines, natural gasolines, and in heavier hydrocarbon distillates including, for example, kerosene and fuel oil.
These mercaptan components are objectionable mainly because of their strong odor, but also, in some cases, due to their objectionable chemical reaction with other hydrocarbons or fuel system components.
There have been many attempts to provide processes for the removal or conversion of mercaptans. Some of the earliest processes included treatment of the hydrocarbon fraction with caustic, clays, and hydrotreating. A significant improvement in the treating of hydrocarbon fractions was made when the UOP Merox Process was announced to the industry in 1959. The Oil and Gas Journal, in the Oct. 26, 1959 edition, contains a discussion of the Merox Process, and also of some prior art processes. The above-mentioned article is hereby incorporated by reference. This process used a catalyst which was soluble in caustic, or alternatively held on a support, to oxidize mercaptans to disulfides in the presence of oxygen and caustic.
In U.S. Pat. No. 3,108,081, there is disclosed a catalyst comprising an adsorptive carrier and a phthalocyanine catalyst for the oxidation of mercaptans. The teachings of this patent are incorporated by reference. This patent taught that a particularly preferred phthalocyanine was the sulfonated derivative, with the monosulfonate being especially preferred.
In commercial operation, a number of catalyst poisons or other deleterious materials are present in the hydrocarbon feed to the processing units provided for mercaptan removal or conversion. Trace amounts of acidic components such as carbon dioxide and H.sub.2 S are frequently encountered.
Many refinery processes produce light gases which contain acid gases such as CO.sub.2, COS and H.sub.2 S and alkyl mercaptans. These processes include crude units, coking units, and fluid catalytic cracking units. Typically gas streams from these units are burned as fuel in the refinery or used in other process units. In the future, more stringent environmental regulations and the introduction of such streams into sensitive downstream processing systems will require the removal of sulfur compounds such as COS, H.sub.2 S, and alkyl mercaptans from such streams.
Accordingly, the treating arts have developed a number of ways of handling these materials. One way is to simply provide a large vessel, termed a "pre-wash," partially filled with dilute aqueous caustic, disperse the hydrocarbon containing trace acidic components into the aqueous caustic, and pass the hydrocarbon stream up through the vessel. Typically the entering hydrocarbon stream will enter the pre-wash vessel through a series of nozzles to insure that there is intimate contact of hydrocarbon with dilute caustic. Sometimes contact is obtained by circulating the caustic inventory with a pump to mix the caustic with entering hydrocarbon in the piping. The strength and quantity of the caustic solution used are generally adjusted so that very little of the weakly acidic mercaptans in the feed are absorbed by the caustic. Only the more acidic carbon dioxide, H.sub.2 S and other trace acidic compounds are removed by the caustic pretreatment. When very low acid contents in the product are required, the pre-wash vessel may be followed by a sand filter coalescer which will remove entrained droplets of aqueous salts from the hydrocarbon stream being treated. However, a sand filter requires frequent attention to maintain its coalescing efficiency and sand is subject to chemical attack by basic aqueous solutions. Furthermore, the presence of both H.sub.2 S and carbon dioxide in the hydrocarbon stream can affect the degree to which the caustic solution is consumed. The use of a caustic solution in the presence of CO.sub.2 will be effective in preventing the breakthrough of the CO.sub.2 up to the point when the caustic is 35-45% spent. Thus, there is a need for a process which improves the chemical utilization of the caustic and thereby reduces the requirements for disposal of spent caustic solution.
Other chemical processes for the treatment of hydrocarbon feeds containing sulfur compounds and acidic components have involved purely chemical reactions such as scrubbing with mono- or diethanolamine or countercurrent extraction using a hot potassium carbonate solution, and chemisorption methods in which iron oxide sponge or zinc oxide reacts with the sulfur compounds to form iron sulfide and zinc sulfide, respectively. A widely used chemical system for treating natural gas streams involves scrubbing with mono- or diethanolamine. The natural gas is passed through the amine solution which absorbs the hydrogen sulfide. The solution from the absorption equipment is passed to a stripping column where heat is applied to boil the solution and release the hydrogen sulfide. The lean, stripped solution is then passed to heat exchangers, and returned to the absorption equipment to again absorb hydrogen sulfide gas. The principle disadvantages of the amine system are its high operating cost, the corrosive nature of the absorbing liquid, its inability to remove mercaptans and water from gas streams, as well as its general inability to selectively remove hydrogen sulfide from carbon dioxide containing streams.
Selective physical adsorption of sulfur impurities on crystalline zeolite molecular sieves is another method for removing mercaptans and sulfur compounds from hydrocarbon streams. Both liquid phase and vapor phase processes have been developed. As used herein, a "physical adsorbent" is an adsorbent which does not chemically react with the impurities that it removes. A typical extraction process for the removal of sulfur from a hydrocarbon stream using a physical adsorbent comprises passing a sulfur-containing hydrocarbon stream through a bed of a molecular sieve adsorbent having a pore size large enough to adsorb the sulfur impurities, recovering the non-adsorbed effluent hydrocarbon until a desired degree of loading of the adsorbent with sulfur-containing impurities is obtained, and thereafter purging the adsorbent mass of hydrocarbon and regenerating the adsorbent by desorbing the sulfur-containing compounds therefrom. A patent to Collins (U.S. Pat. No. 3,654,144) is a representative example of this approach. In these processes the removal of CO.sub.2 is not required to enhance sulfur removal from the feedstream.
Traces of carbon dioxide can be separated from hydrocarbon streams using zeolites having pore sizes in the range of 3 to 4 Angstroms. Typically, calcium zeolite A and treated alumina are used to remove carbon dioxide from hydrocarbon streams which contain ethylene, ethane and propane. Although these adsorbents efficiently adsorb carbon dioxide, they also strongly adsorb ethylene and propane. Fuel gases from refinery processes typically contain significant quantities of ethylene which reduce the effectiveness of calcium zeolite A and treated alumina in treating refinery fuel gases.
A patent to Chao et al. (U.S. Pat. No. 4,935,580) discloses the discovery that a natural clinoptilolite which has been ion-exchanged with metal cations selected from Li, Na, K, Ca, Mg, Ba and Sr is effective in the selective adsorption of minor amounts of CO.sub.2 from light hydrocarbons having 1 to 5 carbon atoms. The method for separation using this adsorbent is especially useful for removal of traces of carbon dioxide from methane or other hydrocarbons. The clinoptilolite also removes water from the hydrocarbon. Chao (U.S. Pat. No. 4,964,889) discloses the use of the clinoptilolite adsorbent for the removal of nitrogen from methane in enhanced oil and gas recovery. Chao further discloses the use of the adsorbent in the separation of carbon monoxide from methane-containing reformer effluents. Chao and Rastelli (U.S. Pat. No. 4,935,580 and U.S. Pat. No. 5,045,515) disclose the use of the ion-exchanged natural clinoptilolite in the separation and removal of traces of carbon dioxide from ethylene or propylene used in the preparation of polyethylene or polypropylene. Chao and Rastelli also disclose the use of the adsorbent for the removal of carbon dioxide from methane or other hydrocarbons, for example, in steam reforming and butanes or butenes.
As environmental restrictions on sulfur emissions to the atmosphere and the disposal of liquid streams from the operation of petroleum refineries are made more stringent, there is a need for a process which will permit the continued use of light gases as fuels and other process uses within the refinery. This invention provides a means for the continued use of these light gases by the removal of mercaptan and sulfur compounds and by reducing a potential liquid stream disposal problem by minimizing the amount of spent alkaline solution created from the neutralization of acid gases and extraction of mercaptans.